462 Mr. C. Wachsmuth on the Paleozoic Crinotds. 
which differs so widely from all other known. Crinoids by its 
distinet bilateral symmetry and unique structure that it forms 
evidently a very distinct group by itself. There may be still 
others, differing in their summit-structure from the general 
plan; but I have yet to discover a single Paleeozoic genus in 
which a special oral aperture has been identified, or in which 
the existence of a solid vault has been disproved or cannot be 
traced by analogy. ‘Thus it may be possible that the solid 
vault was essential under the conditions which prevailed in the 
earlier geological ages. 
Closely related as the recent Crinoids are to their Paleozoic 
ancestors in some points, the solid vault of the latter cannot in 
the remotest degree be homologized with the soft peristome of 
the former. The solid dome forms, as I think I have proved, 
a continuation of the radial and interradial series of the dorsal 
side, and serves merely as a covering and protection for the 
organs underneath. It is in every sense of the word aboral, and 
forms a part of the abactinal system, which, being already re- 
duced in the Pentacrinide and Comatulide to a narrow 
tentacle-furrow, recedes in the Paleeozoic Crinoids one step fur- 
ther, and disappears within the solid walls of the body. The 
actinal system here consists externally only of the arm-furrows, 
whence it continues underneath the vault. These Crinoids, 
therefore, are evidently of lower development and belong to 
an inferior type. 
The ventral peristome of the recent Crinoids serves as a 
madreporic apparatus, introducing the necessary water for 
respiration. It is capable of expansion, and does expand 
when water or food is introduced into the inner cavity, and 
contracts when refuse matter is expelled. These are functions 
which the solid vault could not have performed; and there 
must have been, consequently, important modifications in the 
internal economy of these animals. Comparing the large size 
of the calyx of the earlier Crinoids with the small cup and 
large long arms of the recent types, we find in the former an 
approach to the Cystideans, as also a striking resemblance to 
the nascent Pentacrinus betore its arms are fully developed. 
In the older forms, the radial plan is almost overshadowed by 
the bilateral arrangement of the vault, which reminds of 
the bilateral symmetry in the earlier stages of other Echino- 
derms. All these facts tend to prove that the Paleozoic Cri- 
notds, embracing therein all true Crinotds in which the actinal 
side is closed, represent the young stage of growth of the living 
types. They bear evidently the same relation to the Penta- 
crinide and Comatulide as the Perischoechinide bear to the 
Echini, as the Cystidew and Blastoideee bear to the Paleozoic 
